Inverter control apparatus

ABSTRACT

In an inverter control apparatus including a pulse width modulation inverter supplied with power from a d.c. power source to generate an a.c. output, a frequency control unit controlling the output frequency of the inverter, and a voltage control unit controlling the output voltage of the inverter, a frequency adjusting unit is provided to adjust the operating frequency of the inverter so that the product of voltage and time in each positive half cycle of the output voltage of the inverter becomes equal to that in the next adjacent negative half cycle of the output voltage of the inverter. When the inverter is used in an apparatus for controlling an induction motor, the frequency adjusting unit adjusts the output frequency of the inverter according to the result of detection of a ripple factor of an input voltage of the inverter, and a frequency adjusting-factor correcting unit corrects the frequency adjusting factor of the frequency adjusting unit according to at least one of the output of an induction-motor rotation frequency detector and the output of the voltage control unit.

BACKGROUND OF THE INVENTION

This invention relates to an inverter control apparatus, and moreparticularly to an apparatus suitable for controlling an inverter whichconverts a d.c. output voltage of an a.c.-d.c. converter into a variablea.c. voltage having a variable frequency.

Japanese Patent Publication No. 61-48356 (1986) is known as one of priorart publications disclosing a control method of this kind. JapanesePatent Publication No. 41-48356 notices the fact that, when an a.c.-d.c.converter converts an a.c. voltage into a d.c. voltage, and the d.c.output voltage of the converter is applied to a pulse width modulationinverter to be converted into a variable a.c. voltage having a variablefrequency (VVVF), the output voltage of the inverter pulsates, and,especially, a beat phenomenon occurs at a specific output frequency ofthe inverter, because the output voltage of the a.c.-d.c. converter,that is, the input voltage of the inverter includes a pulsatingcomponent (rectification ripples occurred during rectification).According to the method disclosed in Japanese Patent Publication No.61-48356 which solves the above problem, the ratio between the amplitudeof a sine wave signal and that of a carrier signal of triangularwaveform, that is, the pulse width of a PWM signal is adjusted to dealwith a variation of the inverter input voltage, so that the inverteroutput voltage can be freed from any variation.

Also, JP-A-57-52383 proposed to attain the same object discloses amethod of controlling a PWM inverter in which a pulse processingtechnique is used so as to adjust the pulse width of the PWM signal todeal with a variation of the inverter input voltage.

However, these prior art control methods have such a disadvantage thatthe desired control is not applicable to a voltage range in which theoutput voltage of the inverter attains its maximum level and any furthervoltage control is impossible. That is, the desired control is notapplicable because the number of pulses included in one cycle of theinverter output voltage is only one, and the inverter output voltage ismaintained constant at its maximum level.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an apparatus forcontrolling an inverter so as to minimize an undesirable beat phenomenonof an inverter output voltage attributable to a pulsating componentincluded in an inverter input voltage.

In one aspect of the present invention, the operating frequency of theinverter is adjusted in a way in which the product of voltage and timein one half cycle of the inverter output voltage becomes equal to thatin the next adjacent half cycle of the inverter output voltage.

In a preferred embodiment of the inverter control apparatus according tothe present invention, the slip frequency of an associated inductionmotor is controlled so as to change the output frequency of theinverter, thereby adjusting the operating frequency of the inverter in away in which the product of voltage and time in one half cycle of theinverter output voltage becomes equal to that in the next adjacent halfcycle of the inverter output voltage.

Thus, an unbalance between the adjacent positive and negative halfcycles of the inverter output voltage attributable to the pulsatingcomponent included in the inverter input voltage can be greatlydecreased so as to minimize the beat phenomenon of the inverter outputvoltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block circuit diagram of an embodiment of the invertercontrol apparatus of the present invention when used to control aninduction motor.

FIGS. 2 to 11 illustrate the operation of the embodiment of the presentinvention shown in FIG. 1 in which:

FIGS. 2A, 2B and 2C illustrate the principle of pulse width modulationby comparison between sine wave signals and a triangular wave signal;

FIG. 3 shows the number of pulses and a corresponding inverter outputvoltage relative to a reference frequency command for the outputfrequency of the inverter;

FIGS. 4A, 4B and 4C show the relation between the waveform of theinverter input voltage and that of the inverter output voltage.

FIGS. 5A, 5B, 5C and 5D illustrate how a beat phenomenon of the inverteroutput voltage is suppressed;

FIGS. 6A, 6B and 6C show simulated waveforms of the current and torqueof the induction motor;

FIGS. 7A and 7B illustrate the definition of symbols relating to thecurrent and torque of the induction motor;

FIG. 8 shows the results of simulation of the peak current of theinduction motor;

FIG. 9 shows the results of simulation of pulsation of the torque of theinduction motor;

FIG. 10 is a circuit diagram showing the practical structure of thedetectors 141 and 142 used for detecting the pulsating component andd.c. component respectively of the inverter input voltage; and

FIG. 11 shows the gain and phase characteristics of the detector usedfor detecting the pulsating component of the inverter input voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block circuit diagram of a preferred embodiment of theinverter control apparatus when used to control an induction motor.

Referring to FIG. 1, an a.c. voltage supplied from an a.c. power source1 is converted by an a.c.-d.c. converter 2 into a d.c. voltage, and thed.c. output voltage of the converter 2 is smoothed by a filter capacitor3. A pulse width modulation inverter 4 converts the smoothed d.c. inputvoltage into a variable a.c. voltage having a variable frequency. Theinverter 4 is composed of control switching elements UP to WN which maybe GTO thyristors. An induction motor 5 is energized by the inverter 4.A modulation unit 7 includes a carrier generator 71, a modulation wavegenerator 72, a comparator 73 and a number-of-pulse selector 74. Theoutput of the modulation unit 7 is applied through a gate signalprocessing circuit 6 to the inverter 4 to turn on-off the controlswitching elements UP to WN of the inverter 4 according to apredetermined sequential order.

In FIG. 1, a rotation frequency detector 8 detects the rotationfrequency fn of the induction motor 5. An adder/subtracter 9 adds a slipfrequency command fs to the detected value of the rotation frequency fnwhen the induction motor 5 is in its power running mode, but substractsthe slip frequency command fs from the detected value of fn when theinduction motor 5 is in its regenerative mode. The resultant output ofthe adder/subtracter 9 provides a reference frequency command fo(=fn±fs) for the output frequency of the inverter 4. A current detector10 detects the value Im of current of the induction motor 5, and thisdetected current value Im is compared with a current command Ip in acomparator 11 which finds the difference between Im and Ip. A slipfrequency control unit 12 controls the slip frequency command fs on thebasis of the detected difference between Im and Ip.

The reference frequency command output fo of the adder/subtracter 9commanding the output frequency f of the inverter 4 is applied to themodulation unit 7. In response to the application of the referencefrequency command fo to the modulation unit 7, the modulation wavegenerator 72 generates U-phase, V-phase and W-phase sine wave signals asshown by (b), (c) and (d) respectively in FIG. 2A, and the carriergenerator 71 generates a triangular wave signal as shown by (a) in FIG.2A. The comparator 73 compares the sine wave signals with the triangularwave signal and generates pulses as shown in FIG. 2B. These pulses areused to trigger the control switching elements UP, VP and WP. Pulsesapplied to trigger the control switching elements UN, VN and WN haveinverted waveforms of those shown in FIG. 2B.

Suppose that the input voltage E of the inverter 4 includes its d.c.component Eo only and does not include any pulsating component ΔEo. Inthis case, the U-V output voltage of the inverter 5 has a waveform asshown in FIG. 2C, and there is no unbalance between the positive andnegative half cycles of the output voltage. The output voltage of theinverter 4 is controlled by controlling the width θc shown in FIG. 2B,that is, by controlling the peak value of the sine wave signals shown inFIG. 2A. Further, the number of pulses included in each half cycle ofthe output frequency f of the inverter 4, which frequency f is now equalto the output frequency fo of the adder/subtracter 9, is controlled bythe number-of-pulse selector 74 which changes over the ratio between thefrequency of the triangular wave signal and that of the sine wavesignals, that is, the frequency of the triangular wave signal shown inFIG. 2A. In the example shown in FIG. 2C, the number of pulses is three.FIG. 3 shows the relation between the number of pulses and the output foof the adder/subtracter 9 providing the reference value of the outputfrequency f of the inverter 4. The number-of-pulse selector 74 changesover the number of pulses in the order of, for example, 27-15-9-5-3-1 asshown in FIG. 3. A voltage control unit 13 calculates the ratio betweenthe peak value of the sine wave signals and that of the triangular wavesignal shown in FIG. 2A. That is, the voltage control unit 13 calculatesthe modulation factor β to control the peak value of the sine wavesignals, so that, as shown in FIG. 3, the output voltage V_(M) of theinverter 4 changes continuously relative to the output fo of theadder/subtracter 9 providing the reference value of the output frequencyf of the inverter 4. It will be seen in FIG. 3 that the output voltageV_(M) of the inverter 4 shows a jump when the number of pulses ischanged over to one from three. This is because a certain length of timeis required until the control switching elements UP to WN are completelyturned off, and, therefore, the number of pulses cannot be continuouslycontrolled until the width θc shown in FIG. 2B becomes zero, that is,until the output voltage V_(M) of the inverter 4 attains its maximumlevel where the number of pulses is one.

Even when the filter capacitor 3 for smoothing the d.c. voltage isconnected to the output of the converter 2, the input voltage E of theinverter 4 includes a pulsating component ΔEo attributable to ripplesappearing during rectification. Although this pulsating component ΔEocan be suppressed by increasing the capacity of the filter capacitor 3,it cannot be completely removed. The increase in the capacity results ina correspondingly large size of the filter capacitor 3. Therefore, whenthe pulsating component Eo is taken into account, the input voltage E ofthe inverter 4 is expressed as E (=the d.c. component Eo+the pulsatingcomponent ΔEo), the relation between the input voltage E and the outputvoltage (the line voltage) of the inverter 4 is as shown in FIGS. 4A to4C. In FIGS. 4A to 4C, it is supposed that the modulation factor γ(=theratio between the peak value of the sine wave signals and that of thetriangular wave signal) is γ≧1 in FIG. 2A, and the commanded outputfrequency f of the inverter 4 is equal to the output fo of theadder/subtracter 9. FIG. 4A shows the relation between the input andoutput voltages of the inverter 4 when the frequency fe of the pulsatingcomponent ΔEo (which frequency is constant as it is attributable toripples occurring during rectification) is higher than the output fo ofthe adder/subtracter 9. FIG. 4C shows the above relation when thefrequency fe of the pulsating component ΔEo is lower than the output foof the adder/subtracter 9. In each of FIGS. 4A and 4C, there issubstantially no unbalance between the positive and negative half cyclesof the output voltage of the inverter 4. The condition that thefrequency fe of the pulsating component ΔEo is higher than the output foof the adder/subtracter 9 appears when the rotation speed of theinduction motor 5 is in its low speed range, and the number of pulses isgenerally large as will be seen in FIG. 3. The fact that no unbalanceoccurs between the positive and negative half cycles of the outputvoltage of the inverter 4 even in such a case can be readily understoodfrom FIG. 4A. FIG. 4B shows the relation between the input and outputvoltages of the inverter 4 when the frequency fe of the pulsatingcomponent ΔEo is approximately equal to the output frequency f of theinverter 4 (=the output fo of the adder/subtracter 9). In this case, anunbalance occurs between the positive and negative half cycles of theoutput voltage of the inverter 4. The magnitude of this unbalancechanges in proportion to the difference between the frequency fe of thepulsating component ΔEo and the output frequency f of the inverter 4(=the output fo of the adder/subtracter 9). That is, a beat phenomenonoccurs in the output voltage of the inverter 4.

A frequency adjusting unit 14 is provided to adjust the output frequencyf of the inverter 4. The frequency adjusting unit 14 includes a d.c.component detector, 142 detecting the d.c. component Eo of the inputvoltage, E of the inverter 4 and a pulsating or ripple componentdetector 141 detecting the pulsating or ripple component ΔEo of theinput voltage E of the inverter 4 with a predetermined phase differenceα. The output ΔEo' (|Eo'|=|ΔEo|) of the detector 141 is divided in adivider 143 by the output Eo of the detector 142. Then, the output ofthe divider 143 is multiplied in a multiplier 144 by the output fo ofthe adder/subtracter 9, and an adjusting factor Δfo (=ΔEo' fo/Eo) foradjusting the output frequency f of the inverter 4 appears from thefrequency adjusting unit 14.

The output Δfo of the frequency adjusting unit 14 adjusting the outputfrequency f of the inverter 4 is added in an adder 15 to the output foof the adder/subtracter 9 to provide the frequency command f (=fo+Δfo)commanding the output frequency of the inverter 4. Suppose that theripple factor of the input voltage E of the inverter 4 is K, and thepulsating component ΔEo of the inverter input voltage E pulsates in asinusoidal fashion with the frequency fe. Then, the input voltage E ofthe inverter 4 and the frequency command f commanding the outputfrequency of the inverter 4 are expressed as follows, respectively:

    E=Eo+ΔEo=Eo+KEo sin (2πf.sub.et                   (1) ##EQU1##

When the frequency command f for the output frequency of the inverter 4is applied to the modulation unit 7, the modulation wave generator 72generates U-phase, V-phase and W-phase modulation wave signals G_(U),G_(V) AND G_(W) expressed as follows respectively: ##EQU2##

In the equation (3), γ designates the modulation factor (the ratiobetween the peak value of the modulation wave signals and that of thecarrier signal).

FIG. 5A shows the waveform of the input voltage E of the inverter 4,FIG. 5B shows the waveform of the adjusting factor Δfo for adjusting theoutput frequency f of the inverter 4, and FIG. 5C shows the relationbetween the outputs G_(U) and G_(V) of the modulation wave generator 72.In FIGS. 5A, 5B and 5C, it is supposed that the frequency fe of thepulsating component ΔEo of the input voltage E of the inverter 4 isequal to the output fo of the adder/subtracter 9, and the phasedifference α between the pulsating component ΔEo and its detected valueΔEo' (|ΔEo'|=|ΔEo|) is α=0°. It will be especially seen in FIG. 5C that,by the function of the inverter output frequency adjusting factor Δfoincluded in the second member of the equation (4), the outputs G_(U) andG_(V) of the modulation wave generator 72 are now represented by thesolid waveforms instead of their previous waveforms represented by thedotted curves. FIG. 5D shows the waveform of the U-V output voltage ofthe inverter 4 when the number of pulses is one, that is, when themodulation factor γ(=the ratio between the peak value of the sine wavesignals and that of the triangular wave signal in FIG. 2A) is γ≧1. Itwill be seen in FIG. 5D that the inverter output voltage which has beenpreviously represented by the dotted waveform is now represented by thesolid waveform, and an unbalance between the positive and negative halfcycles of the output voltage of the inverter 4 is greatly decreased.

The amount of unbalance between the positive and negative half cycles ofthe output voltage of the inverter 4 will be numerically discussed withreference to FIGS. 5A to 5D.

Referring to FIG. 5C, when the outputs of the modulation wave generator72 are represented by the dotted waveforms, G_(U) ' and G_(V) ', theseoutputs G_(U) ' and G_(V) ' become zero at times T_(U) ' and T_(V) 'which are expressed as follows respectively: ##EQU3## When the outputsof the modulation wave generator 72 are represented by the solidwaveforms, these outputs G_(U) and G_(V) become zero at times T_(U) andT_(V) which are expressed as follows respectively: ##EQU4## From theequations (3) to (6), ΔT_(U) and ΔT_(V) in the equation (6) areexpressed as follows respectively: ##EQU5## Suppose that the outputfrequency of the inverter is not adjusted by addition of the adjustingfactor Δfo to the output fo of the adder/subtracter 9. That is, supposethat the modulation wave generator generates outputs G_(U) ' and G_(V) 'shown by the dotted waveforms in FIG. 5C, and the inverter 4 generatesthe corresponding output voltage shown by the dotted waveform in FIG.5D. In this case, the product ET' of voltage and time in one half cycleof the output voltage of the inverter 4 is given by the definiteintegral of the equation (1), as follows: ##EQU6## These equations (8)and (9) show that the amount of unbalance ##EQU7## between the positiveand negative half cycles of the output voltage of the inverter 4 has amagnitude |K'| when the output fo of the adder/subtracter 9 is close tothe frequency fe of the pulsating component ΔEo of the input voltage Eof the inverter 4, and a beat phenomenon fluctuating with the frequency(fo-fe) occurs. Even if this magnitude K' may be small relative to the##EQU8## of the equation (8), an excessively large current will flowinto the induction motor 5 resulting in commutation failure or breakdownof the inverter 4, or the torque of the induction motor 5 will greatlypulsate, because the impedance of the induction motor 5 is small whenthe value of the frequency (fo-fe) is small.

On the other hand, when the frequency adjusting unit 14 for adjustingthe output frequency of the inverter 4 is provided, that is, when themodulation wave generator 72 generates outputs G_(U) and G_(V) shown bythe solid waveforms in FIG. 5C, and the inverter 4 generates thecorresponding output voltaqe shown by the solid waveform in FIG. 5D, theproduct ET of voltage and time in one half cycle of the output voltageof the inverter 4 is given by ##EQU9## When the phase difference αbetween the pulsating component ΔEo of the input voltage E of theinverter 4 and the output frequency adjusting factor Δfo is α=0°, thesecond member and third member of the equation (10) cancel each other,and the equation (10) is expressed as ##EQU10## Thus, the amount ofunbalance ##EQU11## between the positive and negative half cycles of theoutput voltage of the inverter 4 is equal to zero, and an undesirablebeat phenomenon of the output voltage of the inverter 4 is suppressed.

In an electric railway car using an inverter, the inverter is saturatedwith its maximum voltage at a speed about 1/2 of the rated speed of thecar, and the frequency only is controlled at higher speeds, in order toenhance the voltage withstand capability of GTO thyristors constitutingthe inverter. Therefore, at speeds higher than about 1/2 of the ratedspeed of the car, the inverter is placed in the stage of single pulsecontrol where adjustment of the output voltage of the inverter isimpossible. On the other hand, the output frequency of the inverter iscontinuously changed over the entire speed range of the car. Therefore,when the a.c. power source 1 shown in FIG. 1 supplies an a.c. voltage ofsingle phase having a frequency of 50 Hz, the frequency of ripplesproduced during rectification in the converter 2 is 100 Hz. In the speedrange in which the output frequency of the inverter 4 passes thisfrequency of 100 Hz, the inverter 4 has already been placed in the stageof single pulse control (fo≧fol in FIG. 3).

By application of the aforementioned principle of the present inventionto such a case, the beat phenomenon between the frequency of ripplesincluded in the output of the converter 2 and the frequency of theoutput of the inverter 4 can be effectively suppressed, so that thespeed of the electric railway car using the inverter can be smoothlycontrolled.

In order to confirm the effectiveness of the control according to theembodiment of the present invention described above, a super computerwas used for digital simulation of the performance of the apparatusunder the following conditions. The induction motor 5 had a capacity of130 KW with its voltage rating of 1,100 V, current rating of 86.7 A andfrequency rating of 75 Hz, and its slip frequency command fs wasmaintained constant at 3 Hz. The input voltage E of the inverter 4 wasexpressed by the equation (1) in which the d.c. component Eo, the ripplefactor K and the frequency fe of the pulsating component ΔEo were 1,500V, 6% and 100 Hz respectively.

FIGS. 6A to 6C show the results of the digital simulation when thereference frequency command fo for the output frequency of the inverter4 was 103 Hz. (The rotation frequency fn of the induction motor 5 wasfn=100 Hz). FIG. 6A shows the result of the digital simulation when theadjusting factor Δfo for adjusting the output frequency of the inverter4 was not used. It will be seen in FIG. 6A that the current of theinduction motor 5 beats greatly with the frequency (fo-fe)=3 Hz due toan unbalance between the positive and negative half cycles of the outputvoltage of the inverter 4 as described already. It will also be seen inFIG. 6A that the torque of the induction motor 5 pulsates greatly withthe frequency fe (=100 Hz) of the pulsating component ΔEo of the inputvoltage E of the inverter 4. FIG. 6B shows the result of the digitalsimulation when the frequency command f for the output frequency of theinverter 4 was adjusted by the output Δfo of the frequency adjustingunit 14 provided for adjusting the output frequency of the inverter 4,and the value of α in the equation (2) was set at α=0°. It will be seenin FIG. 6B that the beat phenomenon of the current of the inductionmotor 5 is substantially eliminated. It will also be seen in FIG. 6Bthat, although the torque of the induction motor 5 pulsates stillslightly, the degree of torque pulsation is greatly decreased ascompared to that shown in FIG. 6A. FIG. 6C shows the result of thedigital simulation when the value of α in the equation (2) was finallyset at α=-5° so as to further decrease the degree of pulsation of thetorque of the induction motor 5. It will be seen in FIG. 6C that thecurrent of the induction motor 5 is substantially beat-free as in thecase of FIG. 6B, and the torque of the induction motor 5 issubstantially free from pulsation. Thus, it has been found that, fromthe aspect of minimizing the pulsation of the torque of the inductionmotor 5, α in the equation (2) is preferably set at a suitable value.

The symbols relating to the current and torque of the induction motor 5are defined as shown in FIGS. 7A and 7B. In FIG. 7A, the peak currentand average torque of the induction motor 5, when no pulsating componentΔEo is included in the input voltage E of the inverter 4, are defined asi_(pn) and T_(av) respectively. On the other hand, in FIG. 7B, anincrement of the peak current of the induction motor 5 and an amount ofpulsation on ripple of the torque of the induction motor 5, when theinput voltage E of the inverter 4 includes a pulsating component Eo, aredefined as Δi_(pb) (=i_(pb) -i_(pn)) and ΔT_(b) respectively. FIGS. 8and 9 show the results of simulation of Δi_(pb) (i_(pn) and ΔT_(b)(T_(av)) respectively when the reference frequency command fo for theoutput frequency of the inverter 4 was set at various values.

It will be seen in FIGS. 8 and 9 that, when the adjusting factor Δfo foradjusting the output frequency of the inverter 4 is not used, theincrement Δi_(bp) (FIG. 8) of the peak current of the induction motor 5and the ripple ΔT_(b) (FIG. 9) of the torque of the induction motor 5become maximum, as shown by two-dot chain curves, at a point where thereference frequency command fo for the output frequency of the inverter4 is approximately equal to the frequency fe (=100 Hz) of the pulsatingcomponent ΔEo of the input voltage E of the inverter 4. The values ofΔi_(pb) and ΔT_(b) are greatly decreased as shown by the one-dot chaincurves in FIGS. 8 and 9 when the frequency command f for the outputfrequency of the inverter 4 is adjusted by the output Δfo of thefrequency adjusting unit 14 adjusting the output frequency of theinverter 4 while setting the value of α in the equation (2) at a =0°.However, in the range where the difference between fo and fe (=100 Hz)is large, the values of Δi_(bp) and ΔT_(b) are slightly larger than whenfo is approximately equal to fe. In order to improve such a situation, aunit 16 is provided for correcting the output Δfo of the frequencyadjusting unit 14 adjusting the output frequency of the inverter 4. Thiscorrecting unit 16 generates an output Kc which is a correctioncoefficient. The output Δfo of the frequency adjusting unit 14 ismultiplied in a multiplier 17 by the output Kc of the correcting unit16, so that the frequency command f for the output frequency of theinverter 4 is now expressed as follows: ##EQU12## Simulation was madeusing various values of Kc in the equation (11) while maintaining α atα=0°. The results of the simulation have proved that the peak currentincrement Δi_(pb) and torque ripple ΔT_(b) of the induction motor 5 canbe improved as shown by the dotted curves in FIGS. 8 and 9 when thefrequency fe of the pulsating component ΔEo of the input voltage E ofthe inverter 4 is divided in a divider 161 by the rotation frequency fnof the induction motor 5, and the output of the divider 161 is squaredin a multiplier 162 as follows:

    Kc=(Fe/fn).sup.2                                           (12)

In order to further improve the torque ripple ΔT_(b) as described withreference to FIGS. 6A to 6C, the value of α in the equations (11) and(12) was changed relative to the reference frequency command fo for theoutput frequency of the inverter 4 as shown in FIG. 9. The results haveproved that the torque ripple ΔT_(b) disappears substantially as shownby the solid curve in FIG. 9. In this case, the peak current incrementΔi_(pb) of the induction motor 5 does not change appreciably as shown bythe solid curve in FIG. 8.

The results of simulation described above have referred to the casewhere the number of pulses in the output voltage of the inverter 4 isone as shown in FIG. 5, and the output (the modulation factor) γ of thevoltage control unit 13 is γ=1. Similar results (effects) are obtainedeven when the number of pulses is larger than one (γ<1). The results ofsimulation in such a case have provded that, when the output of themultiplier 162 is divided in a divider 163 by the modulation factor γ sothat the output (the correction coefficient) Kc of the unit 16correcting the output Δfo of the unit 14 adjusting the output frequencyof the inverter 4 is given by ##EQU13## the values of Δi_(pb) and ΔT_(b)can be more effectively controlled than when Kc is given by the equation(12). When the induction motor 5 is in its starting stage or is rotatingin its low speed range, the value of Kc will become excessively large aswill be readily apparent from the equation (12) and (13). Therefore, itis preferable to provide an upper limit of the value of Kc.

FIG. 10 shows one form of the practical structure of the detector 142detecting the d.c. component Eo of the input voltage E of the inverter 4and that of the detector 141 detecting the pulsating component ΔEo ofthe inverter input voltage E. Referring to FIG. 10, the detector 142detecting the d.c. component Eo of the input voltage E of the inverter 4is in the form of a smoothing circuit including an operational amplifierOP2, resistors Re 21, Re 22, Re 23 and a capacitor C2. The gain (=Re23/Re 21) of the smoothing circuit is unity (1), and the time constant(=Re 23×C2) is selected to be large. On the other hand, the detector 141detecting the pulsating component ΔEo of the input voltage E of theinverter 4 is in the form of a band-pass filter circuit including anoperational amplifier OP1, resistors Re 11 to Re 15, and capacitors C11,C12. FIG. 11 shows the gain and phase characteristics of this band-passfilter circuit 141. Switches S1, S2 and S3 shown in FIG. 10 areselectively turned on depending on the value of the reference frequencycommand fo for the output frequency of the inverter 4 as shown in FIG.11, so that the gain is unity (the value of the input ΔEo≈the value ofthe output ΔEo') at the frequency fe of the pulsating component ΔEo ofthe input voltage E of the inverter 4, and the phase difference αbetween the pulsating component input ΔEo and the output Δfo of the unit14 adjusting the output frequency of the inverter 4 has a value which isappropriate with respect to the reference frequency command fo asdescribed already with reference to FIG. 9.

It will be understood from the foregoing detailed description that theembodiment of the present invention shown in FIG. 1 can suppress a beatphenomenon of the output voltage of the inverter 4 and a beat phenomenonof the current of the induction motor 5 attributable to the pulsatingcomponent ΔEo (the rectification ripples in the output voltage of theconverter 2) included in the input voltage E of the inverter 4.Therefore, any excessively large current does not flow into theinduction motor 5 thereby preventing commutation failure or breakdown ofthe inverter 4, and the torque ripple of the induction motor 5 can alsobe suppressed to ensure smooth operation of the induction motor 5.

The embodiment shown in FIG. 1 has referred to the case where the numberof pulses in the output voltage of the inverter 4 is one as describedwith reference to FIG. 5. However, it is apparent that theaforementioned effects of the present invention are exhibited even whenthe number of pulses is larger than one.

As described above, the present invention can suppress a beat phenomenonof the output voltage of the inverter and a beat phenomenon of thecurrent of the induction motor attributable to the pulsating component(the rectification ripples in the output voltage of the converter)included in the input voltage of the inverter. Therefore, the presentinvention provides the following advantages:

1) Any excessively large current does not flow into the induction motor.

2) Commutation failure or breakdown of the inverter can be prevented.

3) The torque ripple of the induction motor can be suppressed to ensuresmooth operation of the induction motor.

We claim:
 1. An apparatus for controlling an induction motor by aninverter comprising:an a.c.-d.c. converter; a pulse width modulationinverter supplied with power from said converter through a filtercircuit to generate an a.c. output; an induction motor energized by theoutput of said inverter; rotation frequency detecting means fordetecting the rotation frequency of said induction motor; slip frequencycommanding means for commanding the slip frequency of said inductionmotor; frequency control means for controlling the output frequency ofsaid inverter by adding or subtracting the output of said slip frequencycommanding means to or from the output of said rotation frequencydetecting means; voltage control means for controlling the outputvoltage of said inverter according to the output of said frequencycontrol means; means for detecting a ripple factor of the input voltageof said inverter; means for adjusting the output frequency of saidinverter according to the output of said voltage ripple-factor detectingmeans; and frequency adjusting-factor correcting means for correctingthe output of said frequency adjusting means according to at least oneof the output of said rotation frequency detecting means and the outputof said voltage control means; wherein said frequency adjusting meansmultiplies the output of said frequency control means by the output ofsaid voltage ripple-factor detecting means, multiplies then the resultof multiplication by the output of said frequency adjusting-factorcorrecting means, and adds the latter result of multiplication to theoutput of said frequency control means.
 2. An apparatus for controllingan induction motor by an inverter comprising:an a.c.-d.c. converter; apulse width modulation inverter supplied with power from said converterthrough a filter circuit to generate an a.c. output; an induction motorenergized by the output of said inverter; rotation frequency detectingmeans for detecting the rotation frequency of said induction motor; slipfrequency commanding means for commanding the slip frequency of saidinduction motor; frequency control means for controlling the outputfrequency of said inverter by adding or subtraction the output of saidslip frequency commanding means to or from the output of said rotationfrequency detecting means; voltage control means for controlling theoutput voltage of said inverter according to the output of saidfrequency control means; means for detecting a ripple factor of theinput voltage of said inverter; means for adjusting the output frequencyof said inverter according to the output of said voltage ripple-factordetecting means; and frequency adjusting-factor correcting means forcorrecting the output of said frequency adjusting means according to atleast one of the output of said rotation frequency detecting means andthe output of said voltage control means; wherein said frequencyadjusting-factor correcting means includes: dividing means for dividingthe frequency of a rectification ripple component of the input voltageof said inverter by the output of said rotation frequency detectingmeans; and multiplying means for squaring the output of said dividingmeans.
 3. An apparatus for controlling an induction motor by an invertercomprising:an a.c.-d.c. converter; a pulse width modulation invertersupplied with power from said converter through a filter circuit togenerate an a.c. output; an induction motor energized by the output ofsaid inverter; rotation frequency detecting means for detecting therotation frequency of said induction motor; slip frequency commandingmeans for commanding the slip frequency of said induction motor;frequency control means for controlling the output frequency of saidinverter by adding or subtracting the output of said slip frequencycommanding means to or from the output of said rotation frequencydetecting means; voltage control means for controlling the outputvoltage of said inverter according to the output of said frequencycontrol means; means for detecting a ripple factor of the input voltageof said inverter; means for adjusting the output frequency of saidinverter according to the output of said voltage ripple-factor detectingmeans; and frequency adjusting-factor correcting means for correctingthe output of said frequency adjusting means according to at least oneof the output of said rotation frequency detecting means and the outputof said voltage control means; wherein said frequency adjusting-factorcorrecting means includes: first dividing means for dividing thefrequency of a rectification ripple component of the input voltage ofsaid inverter by the output of said rotation frequency detecting means;multiplying means for squaring the output of said first dividing means;and second dividing means for diving the output of said multiplyingmeans by the output of said voltage control means.
 4. An invertercontrol apparatus comprising:a converter for converting an alternatingcurrent to a direct current; a pulse width modulation (PWM) invertersupplied with electric power from said converter; means for commandingan output frequency of said inverter; means for controlling the outputfrequency of said inverter in accordance with a frequency command;voltage control means for controlling an output voltage of said inverterin a variable voltage, variable frequency (VVF) control mode for makingthe output voltage substantially proportional to the output frequency,and in a constant voltage, variable frequency (CVVF) control mode forfixing the output voltage to a substantially constant value; means fordetecting a rectification ripple of a d.c. input voltage of saidinverter; and means for adjusting the output frequency of said inverterin accordance with an output of said rectification ripple detectingmeans in said constant voltage, variable frequency (CVVF) control modeso that the product of voltage and time in each positive half cycle ofthe output voltage of said inverter becomes equal to that in the nextadjacent negative half cycle of the output voltage of said inverter. 5.An inverter control apparatus according to claim 4, wherein saidrectification ripple detecting means includes means for detecting therectification ripple in a frequency band which contains a rectificationripple frequency produced by said converter.
 6. An inverter controlapparatus according to claim 5, wherein said rectification rippledetecting means includes a bandpass filter for passing the rectificationripple in the frequency band which contains the rectification ripplefrequency due to said converter.
 7. An inverter control apparatusaccording to claim 4, wherein said rectification ripple detecting meansincludes means for detecting a ripple rate of the d.c. input voltage. 8.An inverter control apparatus according to claim 7, wherein said outputfrequency adjusting means includes means for adjusting the outputfrequency of said inverter so that the output frequency has a frequencyripple rate corresponding to said ripple rate of the input voltage. 9.An inverter control apparatus according to claim 8, wherein said outputfrequency adjusting means adjusts the output frequency so that saidfrequency ripple rate corresponding to said ripple rate of the inputvoltage is decreased with an increase in a value representing anoperating frequency of said inverter.
 10. An inverter control apparatusaccording to claim 4, wherein said output frequency adjusting meansincludes means for adjusting the output frequency of said inverter inaccordance with an output of said rectification ripple detecting meansin both said variable voltage, variable frequency (VVVF) mode and saidconstant voltage, variable frequency (CVVF) mode.
 11. An invertercontrol apparatus according to claim 4, wherein said rectificationripple detecting means includes means for adjusting a phase differencebetween an input and an output of said rectification ripple detectingmeans in accordance with a value representing an operating frequency ofsaid inverter.
 12. An inverter control apparatus according to claim 4,wherein said voltage control means includes means for controlling theoutput voltage of said inverter in the variable voltage, variablefrequency (VVVF) control mode in a frequency range lower than saidrectification ripple produced by said converter, and in the constantvoltage, variable frequency (CVVF) control mode in a frequency rangeexceeding the frequency range controlled in said VVVF control mode. 13.An inverter control apparatus according to claim 4, furthercomprising:an induction motor energized by said inverter; rotationfrequency detecting means for detecting a rotation frequency of saidinduction motor; slip frequency commanding means for commanding a slipfrequency of said induction motor; and frequency control means forcontrolling the output frequency of said inverter by adding orsubtracting an output of said slip frequency commanding means to or froman output of said rotation frequency detecting means.
 14. An invertercontrol apparatus according to claim 4, wherein said voltage controlmeans includes means controlling the output voltage of said inverter insaid VVVF control mode in a frequency range lower than saidrectification ripple produced by said converter, and in said CVVFcontrol mode in a frequency range exceeding the frequency rangecontrolled in said VVVF control mode.
 15. An inverter control apparatuscomprising:a converter for converting an alternating current to a directcurrent; a pulse width modulation inverter supplied with electric powerfrom said converter; means for commanding an output frequency of saidinverter; means for controlling the output frequency of said inverter inaccordance with a frequency command; voltage control means for saidinverter including pulse number selecting means for selecting the numberof pulses included in a half-cycle of an output voltage of said inverterin accordance with a value representing an operating frequency of saidinverter, said pulse number selecting means selecting one pulse as thepulse number when said operating frequency is equal to or higher than apredetermined frequency (fc); means for detecting a rectification rippleof a d.c. voltage between said converter and said inverter; and meansfor adjusting the output frequency of said inverter in accordance withan output of said rectification ripple detecting means in a control modewherein the pulse number is selected to be one pulse.
 16. An invertercontrol apparatus according to claim 15, wherein said rectificationripple detecting means includes means for detecting the rectificationripple in a frequency band which contains a rectification ripplefrequency produced by said converter.
 17. An inverter control-apparatusaccording to claim 16, wherein said rectification ripple detecting meansincludes a bandpass filter for passing the rectification ripple in thefrequency band which contains the rectification ripple frequency due tosaid converter.
 18. An inverter control apparatus according to claim 15,wherein said rectification ripple detecting means includes means fordetecting a ripple rate of the d.c. voltage.
 19. An inverter controlapparatus according to claim 18, wherein said output frequency adjustingmeans includes means for adjusting the output frequency of said inverterso that the output frequency has a frequency ripple rate correspondingto said ripple rate of the d.c. voltage.
 20. An inverter controlapparatus according to claim 15, wherein said output frequency adjustingmeans adjusts the output frequency so that said frequency ripple ratecorresponding to said ripple rate of the d.c. voltage is decreased withan increase in a value representing an operating frequency of saidinverter.
 21. An inverter control apparatus according to claim 15,wherein said output frequency adjusting means includes means foradjusting the output frequency of said inverter in accordance with anoutput of said rectification ripple detecting means when one pulse isselected and when a pulse number other than one pulse is selected. 22.An inverter control apparatus according to claim 15, wherein saidrectification ripple detecting means includes means for adjusting aphase difference between an input and an output of said rectificationripple detecting means in accordance with a value representing anoperating frequency of said inverter.
 23. An inverter control apparatusaccording to claim 15, wherein said pulse number selecting meansincludes means for selecting the number of pulses included in ahalf-cycle of the output voltage of said inverter to be one pulse in afrequency range not lower than the predetermined frequency (fc) which islower than a rectification ripple frequency (fe) produced by saidconverter.
 24. An inverter control apparatus according to claim 15,further comprising:an induction motor energized by said inverter;rotation frequency detecting means for detecting a rotation frequency ofsaid induction motor; slip frequency commanding means for commanding aslip frequency of said induction motor; and frequency control means forcontrolling the output frequency of said inverter by adding orsubtracting an output of said slip frequency commanding means to or froman output of said rotation frequency detecting means.
 25. An invertercontrol apparatus according to claim 24, wherein said pulse numberselecting means includes means for selecting the number of pulsesincluded in a half cycle of the output voltage of said inverter to beone pulse in a frequency range not lower than the predeterminedfrequency (fc) which is lower than a rectification ripple frequency (fe)produced by said converter.
 26. An inverter control apparatuscomprising:a converter for converting an alternating current to a directcurrent; an inverter supplied with electric power from said converter;means for detecting a ripple rate of a d.c. voltage between saidconverter and said inverter; and means for adjusting the outputfrequency of said inverter so that a frequency ripple rate of the outputfrequency is substantially proportional to said ripple rate of the d.c.voltage, said frequency adjusting means being responsive to a valueindicative of an operating frequency of said inverter for adjusting theoutput frequency of said inverter so that said frequency ripple ratewhich is substantially proportional to said ripple rate of the d.c.voltage is decreased with an increase in the output frequency of saidinverter.
 27. An inverter control apparatus comprising:converter forconverting an alternating current to a direct current; an invertersupplied with electric power from said converter; means for detecting arectification ripple of a d.c. voltage between said converter and saidinverter and providing an output indicative thereof; and phase shiftingmeans for providing a selected one of a plurality of different phaseshift values, said phase shifting means shifting a phase of the outputof said rectification ripple detecting means in accordance with theselected one of the plurality of different phase shift values selectedin dependence upon an operating frequency of said inverter; and meansfor adjusting the output frequency of said inverter in accordance withthe output of said phase shifting means.
 28. An inverter controlapparatus comprising:a converter for converting an a.c. voltage to ad.c. voltage by rectification, the d.c. voltage having a pulsatingcomponent as a rectification ripple; a pulse width modulation (PWM)inverter for receiving the d.c. voltage from said converter as an inputvoltage; means for commanding an output frequency of said inverter andproviding a frequency command indicative thereof; means for controllingthe output frequency of said inverter in accordance with the frequencycommand; voltage control means for controlling an output voltage of saidinverter in a variable voltage, variable frequency (VVVF) control modefor making the output voltage substantially proportional to the outputfrequency, and in a constant voltage, variable frequency (CVVF) controlmode for fixing the output voltage to a substantially constant value;means for detecting the rectification ripple of the d.c. voltage betweensaid converter and said inverter and providing an output indicativethereof; and adjusting means responsive to the output of saidrectification ripple detecting means for adjusting the output frequencyof said inverter so as to minimize an undesirable beat phenomenon of theoutput voltage of said inverter due to the rectification ripple of thed.c. voltage in said constant voltage, variable frequency (CVVF) controlmode.
 29. An inverter control apparatus comprising:a converter forconverting an a.c. voltage to a d.c. voltage by rectification, the d.c.voltage having a pulsating component as a rectification ripple; a pulsewidth modulation (PWM) inverter for receiving the d.c. voltage from saidconverter as an input voltage; means for commanding an output frequencyof said inverter and providing a frequency command indicative thereof;means for controlling the output frequency of said inverter inaccordance with the frequency command; voltage control means forcontrolling an output voltage of said inverter in a variable voltage,variable frequency (VVVF) control mode for making the output voltagesubstantially proportional to the output frequency, and in a constantvoltage, variable frequency (CVVF) control mode for fixing the outputvoltage to a substantially constant value; means for detecting therectification ripple of the d.c. voltage between said converter and saidinverter and providing an output indicative thereof; and adjusting meansresponsive to the output of said rectification ripple detecting meansfor adjusting the output frequency of said inverter so as to minimize anundesirable beat phenomenon of the output voltage of said inverter dueto the rectification ripple of the d.c. voltage in said constantvoltage, variable frequency (CVVF) control mode; wherein said adjustingmeans includes means for adjusting an operating frequency of saidinverter so that a product of voltage and time in adjacent half cyclesof the output voltage of said inverter become substantially equal to oneanother.
 30. An inverter control apparatus comprising:a converter forconverting an a.c. voltage to a d.c. voltage by rectification, the d.c.voltage having a pulsating component as a rectification ripple; a pulsewidth modulation (PWM) inverter for receiving the d.c. voltage from saidconverter as an input voltage; means for commanding an output frequencyof said inverter and providing a frequency command indicative thereof;means for controlling the output frequency of said inverter inaccordance with the frequency command; voltage control means for saidinverter including pulse number selecting means for selecting the numberof pulses included in a half-cycle of an output voltage of said inverterin accordance with a value representing an operating frequency of saidinverter, said pulse number selecting means selecting one pulse as thepulse number when said operating frequency is equal to or higher than apredetermined frequency; means for detecting the rectification ripple ofthe d.c. voltage between said converter and said inverter and providingan output indicative thereof; and adjusting means responsive to theoutput of said rectification ripple detecting means for adjusting theoutput frequency of said inverter so as to minimize an undesirable beatphenomenon of the output voltage of said inverter due to therectification ripple of the d.c. voltage in a control mode wherein thepulse number is selected to be one pulse.
 31. An inverter controlapparatus comprising:a converter for converting an a.c. voltage to ad.c. voltage by rectification, the d.c. voltage having a pulsatingcomponent as a rectification ripple; a pulse width modulation (PWM)inverter for receiving the d.c. voltage from said converter as an inputvoltage; means for commanding an output frequency of said inverter andproviding a frequency command indicative thereof; means for controllingthe output frequency of said inverter in accordance with the frequencycommand; voltage control means for said inverter including pulse numberselecting means for selecting the number of pulses included in ahalf-cycle of an output voltage of said inverter in accordance with avalue representing an operating frequency of said inverter, said pulsenumber selecting means selecting one pulse as the pulse number when saidoperating frequency is equal to or higher than a predeterminedfrequency; means for detecting the rectification ripple of the d.c.voltage between said converter and said inverter and providing an outputindicative thereof; and adjusting means responsive to the output of saidrectification ripple detecting means for adjusting the output frequencyof said inverter so as to minimize an undesirable beat phenomenon of theoutput voltage of said inverter due to the rectification ripple of thed.c. voltage in a control mode wherein the pulse number is selected tobe one pulse; wherein said adjusting means includes means for adjustingthe operating frequency of said inverter so that a product of voltageand time in adjacent half cycles of the output voltage of said inverterbecome substantially equal to one another.